www.isotopic.co.uk

Summary:
Hundreds of wells have been studied using SrRSA since 1991 (initially as Isotopic Analytical Services (IAS) Ltd and since 2001 as Isotopic Ltd), and it is a routine core analysis reservoir characterization application for many oil companies. We have published a detailed review of the SrRSA method together with case studies (Mearns and McBride, 1999).

Core samples of any vintage can be studied by SrRSA. We have experience of obtaining optimum samples from cores cut under many different conditions.

We have been providing SrRSA data on a routine basis for oil company clients from around the world for 18 years.

PRINCIPLES OF Sr ISOTOPE RESIDUAL SALT ANALYSIS (SrRSA)

SrRSA data are the 87Sr/86Sr isotope ratios of the residual salts (and/or water) extracted from cores. These data reflect the Sr isotopic composition of the water (either the irreducible water in hydrocarbon saturated reservoir rocks or formation waters from water leg cores) present within the stata at the time the core was cut.

The 87Sr/86Sr isotope ratios of formation waters are modified by diagenetic and/or hydrodynamic processes. Once established, differences in 87Sr/86Sr isotope ratios within a given body will tend to be homogenized unless homogenization is prevented by some geological process or feature. Thus, the 87Sr/86Sr isotope ratios of residual salts within a hydrocarbon reservoir or aquifer are an effective means of investigating reservoir or aquifer compartmentalization.

The initial pore waters in sedimentary deposits will be determined by their depositional setting (ie. either meteoric or marine water). Upon subsequent burial, the 87Sr/86Sr isotope ratios of formation waters are likely to be modified by diagenesis (water - rock/mineral interactions) and/or large-scale subsurface fluid flow. Variations in 87Sr/86Sr isotope ratios of formation waters caused by natural radioactive decay (ie. 87Rb -> 87Sr + beta particle) are negligible owing to the extremely low concentrations of Rb and low Rb/Sr ratios in natural waters. Diagenetic reactions are essentially mineral dissolution and precipitation reactions which may alter formation water Sr concentrations and /or 87Sr/86Sr isotope ratios. Subsurface fluid flow may cause waters of differing composition to become inter-mixed on all scales causing 87Sr/86Sr isotope ratio variations. Fluid flow may occur in a sedimentary basin owing to combinations of many processes. It is likely that diagenesis driven by changes in formation pressure, temperature and fluid chemistry is closely inter-related with fluid flow. Because of the complex nature of these processes therefore, it is not possible to model their effects on 87Sr/86Sr isotope ratios in other than very general terms. eg. dissolution of silicate minerals such as detrital feldspars or micas is likely to increase the 87Sr/86Sr isotope ratio of formation waters because these minerals typically have very much higher 87Sr/86Sr isotope ratios than marine or meteoric waters.

Homogenization of 87Sr/86Sr isotope ratios of formation waters

Once variations in 87Sr/86Sr ratios in formation waters have been established, they will tend to become homogenized within a given hydrogeological system with time by mixing processes: small-scale fluid flow; and diffusion. Small-scale fluid flow is unlikely to independently homogenize compositions in the laminar flow regime, but may reduce the length scale of diffusion pathways and thus contribute to homogenization by diffusion (discussed in detail in Mearns and McBride, 1999; Figure 5). Chemical diffusion is driven by concentration gradients and so leads to homogenization of Sr concentrations but not 87Sr/86Sr isotope ratios. Tracer diffusion may be viewed as the exchange of neighbouring atoms by random atomic or molecular movements which will lead to homogenization of isotope ratios. Because it is an exchange process, tracer diffusion involves negligible mass transfer. For this reason, this is considered to be the more rapid process. Increased temperature will increase the rate of all of the above mixing processes.

Variations in 87Sr/86Sr isotope ratio within a given hydrogeological system will tend to be homogenized over geological time unless mixing and/or diffusion within the system is prevented. This may be achieved by some form of "barrier" to diffusion or flow which either completely isolates a part of the system or which increases the tortuosity of the pathway between different parts of the system to an extent that prevents or hinders mixing and/or diffusion .

Depending on their configuration and dimension within the reservoir, geological features such as coal beds, shale or argillaceous facies, pervasive authigenic carbonate cement, sealed faults, tar mats etc. may significantly affect homogenization of 87Sr/86Sr isotope ratios within a given hydrogeological system.

As hydrocarbons migrate into and become trapped in a reservoir compartment, the pore-filling formation waters become progressively displaced by the migrating hydrocarbons. At a certain critical level of hydrocarbon saturation the continuous pore-filling fluid phase must change from continuous formation water to continuous hydrocarbon. At this point, the water phase ceases to take any further part in significant diagenetic or hydrodynamic processes. As a reservoir becomes progressively saturated with hydrocarbon therefore, the 87Sr/86Sr isotope ratio of the irreducible waters may be considered as "fossilized" during reservoir filling. For this reason, the SrRSA profile through a reservoir interval is considered to yield useful information about the hydrocarbon filling history by giving an indication of the changes in formation water chemistry at the hydrocarbon-water contact through time (e.g. Lines and Auld, 2004)

Reservoir compartmentalization

The interpretative principles for SrRSA results are detailed in Mearns and McBride, 1999. Sharp steps or breaks in a SrRSA depth profile may reflect a significant difference in the timing of oil emplacement above and below the break. Where a step or break corresponds with a low permeability interval such as a mudstone, coal or tightly cemented bed the data can be interpreted to indicate that the low permeability interval may have been an up-dip sealing barrier at the time of oil emplacement. Alternatively, a step or reversal in a SrRSA profile can reflect the position of a hiatus in oil filling, for example at a paleo – oil-water contact (paleo-OWC). The timing of oil emplacement above and below a paleo–OWC might also be significantly different, but a paleo-OWC might not necessarily occur at a barrier to vertical connectivity (although it may reflect the spill-point of a barrier to lateral connectivity).

Within a water-leg, it is expected that variations in 87Sr/86Sr isotope ratios would be homogenized in geological time by mixing and diffusion. Therefore, a cored homogeneous water leg would be expected to have relatively invariant SrRSA 87Sr/86Sr isotope ratios. Significant variations in SrRSA 87Sr/86Sr isotope ratios within water leg cores may indicate that homogenization has been prevented. This may be achieved by active or relatively recent fluid movement and/or diagenesis in the aquifer. If significant changes in the profile style and values of 87Sr/86Sr isotope ratios occur across geological features within a cored section, then it may be concluded that the features are barriers which have acted to prevent or restrict homogenization of the water leg at that well location.

Lateral connectivity / compartmentalization of a reservoir may be investigated by comparison of SrRSA depth profiles obtained from cores in adjacent well sections plotted at a common TVDSS depth scale.

Salinity variation

The diagenetic and/or hydrodynamic processes that produce variations in the 87Sr/86Sr isotope ratios of formation waters may also cause variations in the water bulk chemistry and salinity. For this reason, variations in SrRSA 87Sr/86Sr isotope ratios through a reservoir section may also indicate that significant variations in formation water salinity exist. For example, Smalley et al (1995) demonstrate that variations in formation water 87Sr/86Sr isotope ratios (determined from SrRSA and centrifuged waters) correspond with a salinity variation of 80,000ppm through a well in the Machar oil field. It may not always be appropriate therefore, to apply a single salinity (and resistivity) value throughout a whole reservoir section that displays a varying SrRSA profile.

Age dating marine sediment - Strontium Isotope Stratigraphy

Summary:
There is an extensive published literature on global sea water strontium isotope studies which is a relatively long established method. We use the most recent calibration data available (e.g. McArthur et al. 2001 - Look-Up Table Version 4: 08/ 04).

Measured strontium 87Sr/86Sr isotope ratios of samples provide age dates for marine sediments using the global sea water calibration data.

Applicable to core, side wall core or cuttings. We have extensive experience of selecting optimum samples for strontium Isotope Stratigraphy analysis.

Specially developed procedures are used to measure strontium 87Sr/86Sr isotope ratios of carbonate and sulphate minerals to high precision, even in very small samples.

PRINCIPLES OF Sr ISOTOPE STRATIGRAPHY

Strontium Isotope Stratigraphy is performed on unaltered marine precipitates (e.g. carbonate and sulphate) and provides a means of dating and correlating marine strata. The technique is based on two principles:

1. That the 87Sr/86Sr ratio of seawater is uniform at any given time throughout the world’s oceans ;
2. That the 87Sr/86Sr ratio in seawater has varied systematically throughout geological time .

The first principle has been demonstrated for the modern oceans (eg. Burke et al, 1982) and is explained by the long residence time of Sr (5x10E6 years) compared with the relatively rapid mixing time of the oceans (approx. 1000 years).

The second principle was first reported by Peterman et al (1970) who documented the systematic variation in the 87Sr/86Sr ratio of unreplaced calcite fossil shells during Phanerozoic time.

Further work performed by the Mobil Research and Development Laboratories in Dallas greatly increased the database and improved the documentation of the variations of 87Sr/86Sr ratios of the oceans during Phanerozoic time (Burke et al, 1982). Subsequent studies have improved the resolution of the variation, particularly in the Cenozoic and upper Mesozoic and a recent comprehensive review has been performed by McArthur et al (2001). By reference to the calibration data, measured 87Sr/86Sr ratios in samples of unknown age may yield precise dates of precipitation of the marine sedimentary material.

This technique produces objective numerical data which have quantifiable levels of uncertainty. It may complement or provide an alternative to biostratigraphy, producing results unaffected by faunal provincialism, diachronism or facies variations and being applicable to strata without age-diagnostic fossil assemblages.

The method may be applicable to carbonate, phosphate or evaporite minerals which precipitated (biogenically or chemically) directly from seawater. Samples must preserve their original seawater 87Sr/86Sr ratio. The method is not applicable to cements which have undergone diagenetic alteration resulting in modified 87Sr/86Sr ratios.

The technique is most applicable to strata ranging from the Jurassic to Pleistocene because of the almost uninterrupted increase in seawater 87Sr/86Sr ratio during this interval. Its applicability in dating strata of Palaeozoic age is limited by cyclic fluctuations of the seawater 87Sr/86Sr ratio during that era. Cyclic fluctuations may however, be overcome by analysing suites of samples with a range of stratigraphic positions to observe trends of variation with increased depth / age.

Strontium, oxygen and carbon isotope compositions of authigenic carbonate cements provide information on reservoir and basin fluid composition at the time the cement formed.

Isotopic are experienced in the integration of isotope data with the results of specialist petrographic studies.

Profiles of strontium, oxygen and carbon isotope compositions through pervasive carbonate cement intervals in sandstone may help determine origin and geometry of the cemented layers.

PRINCIPLES OF Sr-O-C ISOTOPE DIAGENESIS OF CARBONATES

Delta notation

Stable isotope analysis of carbonates measures the ratio of the heavy to light isotopes of O and C in the mineral species under study (eg. The 18O/16O ratio and the 13C/12C ratio). In order that the results from different laboratories may be compared, these ratios are normalised relative to a fixed standard ratio of the isotopes and such results are conventionally prefixed with the delta symbol. The original standard that was used for carbonate analyses was carbon dioxide prepared from belemnites collected from the Peedee Formation (Cretaceous) of South Carolina, USA. This is known as the PDB standard. Actual material of the PDB is not used now and most labs use commercially available CO2 gas that has been calibrated by analysis of carbonate standards prepared by the American National Bureau of Standards (e.g the NBS 19 marble etc). Using the delta notation, the carbon isotope ratio of a sample is therefore reported as follows:

d13Csample = [(13C/12C)sample – (13C/12C)PDB] / (13C/12C)PDB x 10E3

Thus, if the sample has the same 13C/12C ratio as PDB then it will have a d13C = 0 per mil. Positive d13C values indicate a sample with a higher 13C/12C ratio than PDB and negative d13C values indicate a sample with lower 13C/12C ratios than PDB.

The other widely used standard for oxygen isotope analyses is the mean 18O/16O isotope ratio obtained from ocean water (Vienna Standard Mean Ocean Water – VSMOW, so named as it was agreed at an international meeting to discuss stable isotope standards in Vienna in 1976).

Oxygen isotopes

The measured oxygen isotope composition of a mineral (d18O) is controlled by a temperature dependent fractionation of oxygen between the precipitating water and the mineral (a key reference for isotope systematics is Faure and Mensing, 2005).

The temperature dependence of the calcite - water fractionation used by Isotopic in carbonate diagenesis studies is as follows (using fractionation constants from Longstaffe, 1987):

Therefore, using the measured d18O of the mineral and the appropriate fractionation constants, allows the range of possible precipitating temperatures and precipitating water oxygen isotope compositions to be constrained. Using these data it is possible to use a temperature of precipitation (e.g. from fluid inclusions) to determine the precipitating water oxygen isotope composition. It is also valid to make an assumption / interpretation about the precipitating water oxygen isotopic composition and determine a paleotemperature of cementation.

Carbon isotopes

Because the carbon isotope ratio 13C/12C is measured relative to the PDB standard, other similar primary marine carbonate material would be expected to have a d13C around 0 per mil PDB. However, secular variations within the range 0±5 per mil have occurred throughout geological time (e.g. Veizer et al 1999). Primary variations in d13C of authigenic carbonate cements occur as the result of different chemical processes acting on organic carbon.

For example, sulphate reduction and bacterial degradation of organic matter result in 12C enrichment of associated carbonate. Such processes occur directly below the sediment-water interface in the marine environment. Paragenetically early carbonate with low negative d13C values (e.g. -25 per mil PDB) are characteristic of this process.

Fermentation of low molecular weight acids or reduction of carbon dioxide by methanogenic bacteria can give rise to dissolved inorganic carbon (bicarbonate ions) enriched in 13C. Such processes occur at burial depths of a few tens of metres to approximately 1km. Carbonates with positive d13C (e.g. +10 to +15) are characteristic of these processes.

Authigenic carbonate cements (particularly in sandstones) may yield d13C values intermediate between the extremes outlined above. Assuming the materials analysed are not multi-generation assemblages of cements, this would suggest the incorporation of bicarbonate derived from more than one primary source through dissolution and re-precipitation of earlier cements.

Petrographic study is essential prior to any isotopic work to determine whether mixed generations of cement are present and if so, the appropriate sampling procedure to be adopted (micro-drilling, sequential dissolution etc.).

Strontium isotope ratios (87Sr/86Sr) of carbonate minerals

Strontium is an alkaline earth element that behaves in a similar way to Ca in geological systems. The 87Sr/86Sr isotope ratio therefore, provides a means to characterise and investigate the origins of Ca in carbonate cements. The 87Sr/86Sr isotope ratio of seawater has varied systematically between 0.7068 and 0.7092 throughout Phanerozoic time. Carbonate minerals which precipitate directly from seawater or as a direct result of the dissolution and re-precipitation of marine biogenic carbonate, should exhibit the 87Sr/86Sr ratio of the sea water at the time of their precipitation (the basis of Sr Isotope Stratigraphy). In contrast, later burial diagenetic (mesogenetic) carbonates incorporate 87Sr released during the dissolution of detrital silicate minerals such as feldspars and micas. Such carbonate cements inherit the 87Sr/86Sr ratios of the evolved formation waters from which they crystallised which typically have 87Sr/86Sr ratios greater than contemporaneous seawater.

Combined Sr-O-C data

As outlined above, Sr, O and C isotopes yield different information about the origin of formation water, dissolved ions and temperatures at points in time within reservoir compartments. By measuring combined Sr-O-C isotopes it may be possible therefore, to characterise, identify and correlate discrete generations of carbonate cement within a reservoir sequence in the context of a reservoir characterization and reservoir compartmentalization study. Contemporaneous cement forming in separate reservoir compartments may be petrographically similar but have different Sr-O-C isotope compositions reflecting differences in prevailing sources of water and ions.

One further application is to produce Sr-O-C profiles through carbonate cement intervals to investigate the growth history of the cement. Such information may allow interpretations of the geometry of pervasive carbonate cemented intervals to be made (in conjunction with SrRSA studies). Discrete nodular cement would be expected to have roughly symmetrical Sr-O-C profiles, whereas more laterally extensive layers of cement may be expected to isolate underlying from overlying formation waters thus giving rise to asymmetrical Sr-O-C profiles.

Summary:
There is an extensive published literature on the use of the Samarium-Neodymium isotope system in provenance and correlation studies. We have published a case study in Dalland, Mearns and McBride, 1995.

The measured neodymium isotope ratios (143Nd/144Nd) and / or the Provenance Age calculated from the samarium-neodymium isotope ratios (147Sm/144Nd and 143Nd/144Nd) of samples are used to zone reservoirs and correlate strata.

Samarium – neodymium isotope data from clastic strata can also provide an indication of possible sediment source terrain compositions and hence may contribute to an understanding of sediment provenance.

This technique is applicable to core, side wall core or cuttings samples of sandstone and mudstone facies.

Integration of the results with other chemostratigraphic techniques can provide robust correlations of biostratigraphically barren sequences.

PRINCIPLES OF SAMARIUM – NEODYMIUM ISOTOPE STRATIGRAPHY

Sm and Nd are Rare Earth Elements (REE). During partial melting of the upper mantle, the REEs as a group undergo extensive fractionation between mantle and crust. Crustal rocks are generally more enriched in the light REEs (including Sm and Nd) while the mantle residue is depleted in these elements. From the time that Sm and Nd are fractionated between crust and depleted mantle the 143Nd/144Nd ratio of the crustal component is modified by the long-lived radioactive decay reaction:

147Sm -> 143Nd + 4He (half life = 106 Ga)

Thus, the Sm-Nd isotopic composition of a mantle-derived crustal rock is controlled by the time since the rock first formed from the mantle.

Amongst the properties of REEs are extremely low solubilities in water. Upon weathering of crustal rocks therefore, Sm and Nd are transported in the detrital sediment mass and are not thought to be involved in most early diagenetic processes. Furthermore, because Sm and Nd are geochemically very similar, they probably undergo negligible fractionation between source and sediment in the sedimentary cycle (Mearns, 1988). For these reasons, a clastic sediment is considered to inherit the Sm-Nd isotopic composition of its source terrain(s). Once deposited and buried, the sedimentary rock is regarded as a closed system with respect to Sm-Nd isotopic composition, again owing to the low mobility of REEs in the mesogenetic realm. A sedimentary rock will therefore, always preserve the inherited Sm-Nd isotopic composition of its source terrain(s) regardless of diagenetic processes. Furthermore, sedimentary deposits derived from recycled older sedimentary material will inherit the Sm-Nd isotopic composition of the recycled sediments. i.e. the Sm-Nd isotope signatures of first-cycle sediments are transmitted through subsequent sedimentary cycles.

Fluvial Systems

At any given point in geological time, individual clastic sediment source terrains may have unique Sm-Nd isotopic characteristics which may be transmitted to river sediments. Furthermore, adjacent source terrains may yield sediment with contrasting or similar Sm-Nd isotopic characteristics depending on the regional geology. Downstream from confluences however, the sediment from any number of different source terrains are mixed to give a weighted mean Sm-Nd isotopic composition for the combined source areas. At any point of deposition therefore, sediment may be deposited which has been derived from either a single or from multiple sources. Even in the case of a depositional system supplied by a single source, the isotopic composition of the sediment may vary through time owing to unroofing of different rock types or through expansion/contraction of the source terrains. Thus, these combined processes are likely to lead to variations in Sm-Nd compositions through a fluvial facies sedimentary sequence. It is these variations and their relationship to other geological features that are utilised for correlation by the Sm-Nd isotope technique.

Marine Systems

The likelihood of homogenization of sediment (and Sm-Nd isotope composition) from different clastic source terrains is greatly increased in the marine depositional environment. Thus, variation through a marine sedimentary sequence is expected to be less pronounced than through a fluvial section. Nevertheless, the Sm-Nd isotope technique may still be applicable as a correlation tool in these sequences if sufficient variations in Sm-Nd isotope composition are recorded. For example, in a submarine fan setting, sediment with different Sm-Nd isotope compositions may be transported by separate submarine channel systems. Stacking of such channels or submarine fans may thus result in measureable (and geologically meaningful) variations through a vertical reservoir section.

Provenance

Because the Sm-Nd isotope composition commonly represents the average composition of the source terrain (or terrains) of a sediment, it is seldom diagnostic of a specific provenance (at least in the North Sea basin). It is possible however, that this may not be the case in other depositional basins. Therefore, while changes in Sm-Nd isotope composition may be anticipated and recorded, it is not generally expected that these data will allow the location of actual clastic source areas to be determined.

An exception to this in the North Sea basin is the Upper Triassic - Lower Jurassic, Statfjord Formation of the Snorre and Gullfaks oilfields which contains fluvial facies sandstone members with distinct Sm-Nd isotope compositions which restrict their provenance to specific Erian Shield source terrains (and consequently constrains possible paleogeographies, sediment transport directions etc., Mearns, 1989; Mearns et al, 1989). Another example is the Middle Jurassic Ness and Tarbert Formations of the Gullfaks Field which contain sandstones with distinct Sm-Nd isotope compositions which restrict possible source terrains to areas containing Jurassic or older volcanic rocks (eg Rattray Formation volcanics in the central North Sea; Mearns, 1989, 1992, Dalland et al, 1999). It is possible that similar contrasting young volcanic and ancient Pre-Cambrian source terrains may exist for clastic sedimentary sequences in other depositional basins world-wide (e.g. West Africa).

Monitoring produced waters and investigating water allocation – Water strontium, oxygen and hydrogen isotope composition.

Summary:
The strontium, oxygen and hydrogen isotope compositions of produced water samples may be used to investigate the geological origin of the waters, can assist in reservoir compartmentalization studies (with SrRSA), and can help indentify and interpret causes of changes in water composition with time during production. Taken together, these data may contribute to a better understanding of water allocation.

The isotope geochemical compositions of produced water samples complement the more routine chemical analyses of produced water samples such as cation and anion concentration.

Strontium, oxygen and hydrogen isotope compositions of water are reported as 87Sr/86Sr isotope ratios, d18O VSMOW and d2H VSMOW.

Results are typically reported within 3 to 4 weeks of receipt of samples, with formal reporting carried out in consultation with the client’s needs.

We have been providing specialized isotopic studies for the international oil industry on a routine basis for 18 years. This long term continuity and experience allows us to advise clients on the optimum sample requirements for the analyses, and maintain an overview of consistency and reproducibility of results and interpretations that we provide.

PRINCIPLES OF Sr-O-H ISOTOPE STUDIES OF PRODUCED WATER SAMPLES

Basics

A key reference for isotope studies is Faure and Mensing (2005). Hydrogen has two stable isotopes: 1H and 2H (also known as deuterium). A third isotope of hydrogen (3H or tritium) is unstable (radioactive) and occurs in nature as a result of the interaction of cosmic rays and the atmosphere. The natural abundance of tritium is negligible owing to its short half life. The use deuterium and tritium enriched waters in tracer studies is not discussed in this article which focuses on the isotopic compositions of natural waters.

Oxygen has three stable isotopes: 16O, 17O and 18O. The stable isotopes of oxygen and hydrogen are subject to mass dependant fractionation. The ratios of the isotopes of hydrogen and oxygen (i.e. 2H/1H and 18O/16O) are conventionally normalized relative to Standard Mean Ocean Water (SMOW or Vienna – V - SMOW) and reported as delta values. e.g. a water with the same 2H/1H and 18O/16O isotope ratios as VSMOW will have d2H and d18O values of zero per mil VSMOW. Negative delta values indicate lower isotope ratios compared to VSMOW and positive values indicate higher isotope ratios compared to VSMOW.

Strontium has four naturally occurring stable isotopes: 84Sr, 86Sr, 87Sr and 88Sr. Of these, the isotope 87Sr is formed by the decay of 87Rb. This gives rise to so called radiogenic 87Sr, the abundance of which varies in nature, making the 87Sr/86Sr isotope ratio a useful measurement in the study of various natural systems. The 87Sr/86Sr isotope ratio of a rock or mineral depends on the age and Rb/Sr ratio of the rock or mineral.

The effects of any mass dependent fractionation of strontium isotopes that may occur in natural geochemical processes and within the mass spectrometer during measurements are corrected for by simultaneous measurement of, and normalization to, the stable 86Sr/88Sr isotope ratio which is fixed by international convention as 0.1194 (Steiger and Jæger, 1977).

There is no conventional agreement on the use of delta notation for reporting strontium isotope ratios, and so these results are reported as the measured 87Sr/86Sr isotope ratios. Isotopic always report quality control (QC) standard values obtained during the sample analyses to allow inter-laboratory comparison the results.

Natural Water Sr-O-H Isotope Compositions

Surface waters

Two key water types might be identified as being original, surface waters: meteoric water and sea water. (Juvenile water originating from deep within the mantle may once have occurred at the surface of the Earth during its very early history, but is not currently being discharged by hot springs in oceans or continents).

Meteoric water

The lightest molecules of water (i.e. containing 1H and 16O) evaporate preferentially relative to heavier water molecules (e.g. containing 2H and 18O). On the other hand, the heavier water molecules condense preferentially compared with the lighter molecules. The hydrogen and oxygen isotopic compositions of meteoric precipitation (rain, snow, hail) on the Earth are related to each other as follows:

dD = 8 d18O + 10 (Craig, 1961)

Which is known as the meteoric water line (MWL). Higher latitude meteoric waters have negative d18O and d2H values while lower latitude / equatorial meteoric waters have values approaching 0 per mil VSMOW.

NB. for meteoric waters with d18O = 0 permil VSMOW (i.e. the same as sea water), the corresponding d2H values are in the region of +10 per mil VSMOW. This so called deuterium excess is a characteristic of meteoric waters.

Pure (fresh) water will have only trace to no strontium present. Surface river water may contain very low concentrations of strontium derived from mineral weathering. The 87Sr/86Sr isotope ratio of surface river water may vary widely depending on the age and Rb/Sr ratios of the weathered minerals it has come into contact with.

Meteoric waters may be depositional pore waters in continental facies sediments or may have entered buried strata through exposed outcrops and driven by hydrostatic head.

Sea water

Standard Mean Ocean Water (VSMOW) has d18O and d2H values of 0 per mil VSMOW by definition. It is possible the d18O of the oceans may have been lower and ocean temperatures may have been higher in past geological periods when there were no polar ice caps.

Present day sea water has a strontium concentration of about 8 mg/l and a 87Sr/86Sr isotope ratio of about 0.7092. The strontium isotope composition of sea water has varied systematically through geological time principally as a result of changes in the rate of sea floor spreading and subduction (and the associated volcanic activity, continental weathering, etc). The variations in sea water 87Sr/86Sr through geological time is the basis for the Sr Isotope Stratigraphy method.

Sea water was the original pore water for all marine facies sediments.

Modifications to surface water

Most natural processes like evaporation, isotope exchange with rock minerals under elevated temperature conditions, and mixing of sub-surface brines act to shift the isotopic compositions of surface waters to the right of the MWL.

Evaporation of meteoric or sea water results in evaporative brines with modified oxygen and hydrogen isotopic compositions. Such water may have higher or lower d18O and d2H values compared to meteoric / sea water depending on the degree of evaporation.

Sub-surface brines. Formation waters produced from oil and gas wells

The Sr-O-H isotope composition of a formation water is determined by diagenetic (water-rock) reactions in the sub-surface during burial of the rock sequence, and by mixing of different water compositions in the sub-surface.

Silicate minerals are enriched in 18O relative to VSMOW and have positive d18O values which range from +20 in quartz to +5 in olivines and pyroxenes. Marine carbonate and pelagic clay minerals have d18O values in the order of +15 to +30 per mil VSMOW.

Note, the use of two different normalization values for oxygen isotope values can be confusing. Carbonate mineral d18O values are typically reported relative to PDB, whereas water and silicates are reported relative to VSMOW. The relationship between these is:

So a marine carbonate with d18O = 0 per mil PDB will have a d18O SMOW value of +30.91 per mil.

Strata rich in clay minerals (e.g. shales containing kaolinite, illite and smectite) as well as rocks containing mica or amphiboles are effective sources of hydrogen which is likely to affect the d2H isotope compostion of waters by water-rock interaction. Such rocks / minerals may have low negative d2H values of the order of -40 to -90 per mil VSMOW.

Oxygen and hydrogen isotope exchange reactions occur when water is in contact with silicate or carbonate rocks in the subsurface.

Dissolution of minerals with high Rb/Sr ratio such as feldspars and micas may contribute high 87Sr/86Sr ratios to sub-surface formation waters. Dissolution of minerals with low Rb/Sr ratios such as marine carbonates and juvenile volcanic rocks may contribute Sr with low 87Sr/86Sr ratios to pore waters during diagenesis.

North Sea formation waters have a wide range of d18O (-5 to +9 per mil VSMOW) and d2H (-2 to -45 per mil VSMOW) values which fall to the right of, and below the MWL. Waters from reservoirs in Northern North Sea tend to have more negative d18O than formation waters from the Central North Sea (Warren and Smalley, 1994). Strontium isotope ratios of North Sea formation waters also vary widely from low values associated with volcanic sequences (e.g. 0.705 – 0.706), to high values associated with clastic conglomerate sequences (87Sr/86Sr ratios higher than 0.735).

Mixing of sub-surface waters in response to regional pressure gradients and hydrocarbon migration, mixing with meteoric water during periods of uplift and erosion (e.g. for certain Northern North Sea strata), diagenetic precipitation of authigenic minerals such as calcite, gypsum, quartz etc, diagenetic dissolution of minerals such as feldspars and micas, recrystallization of clay minerals, and isotope exchange reactions between pore waters and host rocks have all contributed to the Sr-O-H isotopic compositions of North Sea formation waters.